In the area relating to clean rooms and the microelectronic industry the removal of Airborne Molecular Contamination (AMC) in the form of Acids, Bases, Condensables, Dopants, Oxidants and Volatile Organic Compounds (VOC) is achieved by molecular filter and filter elements. AMC may cause corrosion of process wafers, circuit boards, tools, instruments etc. AMC can also cause doping errors, nucleation errors, lithography process related defects, hazing of wafers, optics, lenses and numerous other problems that may generate yield losses in the production or damage to the production equipment.
A variety of filtration technologies are available to choose from, but these can generally be divided into five configurations.
Adsorbent powder slurry-coated nonwovens are inexpensive and have low pressure drop, but have poor adsorption performance due to the small amount of adsorbent present and the fact that much of it is covered with adhesive. This type is not typically appropriate for clean room applications.
Bulk adsorbent packed beds and panels are perhaps the best known, and are available in a wide range of price and performance combinations. These are typically large, heavy systems requiring significant labour for change out, but are especially effective for applications with relatively high contaminant levels or when a high filter efficiency must be sustained for a long period of time. However, performance can be quite variable depending on how the filters are manufactured and installed. Dusting, by-pass, and high pressure drop may also be of concern with this type of filter.
Fabrics made entirely from carbon fibres: One major shortcoming with carbon fiber filters is that they are too dirty to be used in an advanced clean room environment without a particle filter downstream, as these fibers shed. Carbon is a conductive material and shedding can lead to severe problems in a facility producing electronic components. Honeycomb filters is another type for which relatively high efficiencies can be reached while keeping a low pressure drop, but have most of the time poor adsorption performance due to the small amount of adsorbent present. Dusting is also a concern with this type of filter.
Adsorbent loaded nonwovens are a relatively new “hybrid” product typically applied in pleated form. With the proper construction and application, these products can offer extremely high value in terms of high efficiency and service life, as well as low pressure drop and ease of handling. Further, pleated filters are more flexible than the honeycomb structures described above: The pleated filters have the simple possibility to vary number of pleats or pleat heights to vary the performance at will. This is impossible with honeycomb structures for which only the height can be adjusted, with pressure drop impact, or the number of cells/cm2, requiring the development of a new expensive production process. It is also much more complicated to vary the type of adsorbent than using a pleated element.
In some applications with high levels of contamination or contamination difficult to remove such as low boiling point VOC, normally a bulk adsorbent packed bed filter would be required to achieve reasonable life/service times. This kind of filters are not accepted inside a clean room (air handling units, fan filter units, minienvironments or tools) because of the pressure drop and energy costs generated. Very often existing clean rooms cannot be retrofitted with these systems without reconstruction and as a result substantial investments. Further, to protect the high level of cleanliness of the clean room, particle filters must be installed in combination with bed filters adding further pressure drop and increasing energy costs. On the other hand the use of smaller and cleaner filters results in a less attractive cost of ownership (CoO) for the customer due to very frequent filter exchanges.
The physico-chemical mechanisms involved in the removal of AMC can be described as following: The gas/vapour molecules diffuse inside the adsorbent, i.e. they move from area of high concentration to low concentration. The rate of diffusion is directly related to the difference between the two concentrations. As the internal surface of the adsorbent gets covered with gas/vapour molecules, the filter efficiency starts to decrease. The properties of the internal surface i.e the active surface groups present and most important the pore size distribution will determine which gas or vapour molecules that are withheld. When the effluent gas/vapour reaches an unacceptable level, the filter needs to be changed. Usually the filter is discarded, and a new relatively expensive filter is installed.
Regenerable filters are known in fixed installations for instance twin beds of adsorbent that are alternately in service or under regeneration. Normally, only one type of adsorbent is used in this type of systems.
WO 0209847 discloses a regenerable air cleaning device with a control system that is adapted to determine when to regenerate the filter. The Activated Carbon Fibers used in that filter will be contaminating the air with fibers and thus can not be used in a clean room environment without the use of a secondary particle filter.
New systems have started to appear that uses a honeycomb structure made of either activated carbon on a paper or ceramic structure or constructed from a base material that can be carbonized and activated as a whole. The U.S. Pat. No. 6,964,695 or EP 04724071 or U.S. Ser. No. 10/344,248, shows a carbon monolith to be regenerated by direct electrical heating. The drawback of this and other honeycomb/monolith based systems are mainly that they have a fixed geometry that only can be adapted to the filtration by elongation in the air direction and thus suffer an increased pressure drop or by enlarging the external dimensions of the actual filter which seldom is acceptable in an already constructed system or in a system trying to used standard parts.
Thus, there exists a need to overcome the problems and shortcomings with the methods and filters used today as mentioned above.